The present invention relates to a sheet feeding device used for an image forming apparatus such as a copying machine, a printer, a facsimile machine, a printing machine and a multifunction peripheral, and in particular, to a sheet feeding device that can separate sheets having a strong sticking power between the sheets such as coated sheet one sheet by one sheet surely, and can feed out them.
On the image forming apparatus, there is provided a sheet feeding device that feeds out sheets with a sheet feeding roller one sheet by one sheet from a bundle of sheets wherein a plurality of sheets are stacked. In the sheet feeding device of this kind, if plural sheets are fed simultaneously on a multi-feeding basis, it causes sheet jamming. Further, if conveyance power is small, it tends to cause misfeeding. Therefore, there has been devised a way to feed out one sheet by one sheet surely. Namely, the misfeeding is prevented by feeding out the uppermost one sheet of the stacked sheets, by making the coefficient of friction between a sheet feeding roller and a sheet. Further, for preventing that two or more sheets are fed out simultaneously, second sheet and thereafter are pushed back through separation operations by means of a separation roller, a separation pad or a separation claw, so that the uppermost one sheet only may be fed out.
This method is effective as far as ordinary sheets only are used. In recent years, however, use application for a copying machine and a printer is expanded, and types of sheets to be used including coated sheet have been diversified. Some of these various types of sheets show strong sticking power between the sheets when they are stacked, and they sometimes make it difficult to prevent multi-feeding surely on the aforesaid sheet feeding device.
Therefore, there has been suggested a method wherein a blowing outlet is provided on the side of the conveyance direction for stacked sheets, so that air may blow from the blowing outlet against plural sheets positioned to be in an upper part of the superposed sheets in their thickness direction, to separate sheets by causing air to pass through clearances between sheets.
However, a coated sheet has a characteristic to stick to other coated sheet firmly under the high humidity environment. A bundle of sheets stuck firmly is a heavy mass for slow wind velocity, thus, it is impossible for soft air blowing from a lateral side to separate stuck sheets and to cause a sheet to rise to the surface.
For solving this problem, it is considered to make the air blowing to raise the sheet to be more powerful. However, even when airflow is made to be more powerful, separation of sheets into a single sheet is not improved, though sheets are pushed up greatly.
With the foregoing as a background, Unexamined Japanese Patent Application Publication No. 60-52428 suggests a constitution having therein a sheet sticking conveyance section that attracts one sheet from a bundle of sheets supported by a sheet superposing device through suction of air to convey the sheet and a blowing section that blows air against an upper portion of the bundle of superposed sheets in the conveyance direction from a leading edge of the sheet. By blowing air in the direction from the front side of the sheet, it is possible to lift several sheets in the bundle of superposed sheets, and to attract the uppermost sheet only with the sheet sticking and conveyance section, to convey. After the uppermost one sheet is attracted by the sheet sticking and conveyance section, air blowing from the front side acts to separate the second sheet and thereafter.
In the sheet sticking and conveyance section of this kind, a pump which is generally for a small capacity and for high pressure is used for suction. However, in case of attracting a thick sheet, a large-sized pump should be used to acquire sufficient airflow. Thus, there have been problems that noise grows greater, and plural sheets are attracted to cause multi-feeding in case of thin sheets.
Therefore, in the constitution in Unexamined Japanese Patent Application Publication No. 60-52428, a pump which is for a large capacity and for low pressure and a pump which is for a small capacity and for high pressure are provided to be used by switching depending on sheet quality, to cope with the aforesaid problems.
However, the method in Unexamined Japanese Patent Application Publication No. 60-52428 has a problem of cost increase, because two sets of pumps are required. Therefore, in Unexamined Japanese Patent Application Publication No. 06-219578, a measuring instrument for measuring suction power and a measuring instrument for measuring blowing power are arranged respectively in a sucking duct and a blowing duct to control rotation of a motor for the sucking duct and to control rotation of a motor for the blowing duct, based on their measured values. By doing this, sucking power and blowing power can be stabilized.
Further, Unexamined Japanese Patent Application Publication No. 60-52429 suggests a constitution having a valve provided in a sucking duct, wherein a period of time from the moment when a sucking device starts its suction up to the moment when a sheet is stuck is measured, and when this period of time is long, sucking power is raised by enlarging an aperture amount of the valve, while, when this period of time is short, sucking power is lowered by lowering the aperture amount of the valve.
Further, Unexamined Japanese Patent Application Publication No. 05-270676 suggests a constitution having a damper in a sucking duct, wherein timing for opening and closing the damper is changed to cope with a size and a thickness of a sheet.
Now, a sheet stuck to a sticking and conveying section is conveyed, and its leading edge is nipped by a conveyance roller, and then, is fed into an image forming section by the conveyance roller. When a size of a sheet is small, it happens that a trailing edge of the sheet is positioned at the middle of the sucking duct, and a part of the sucking duct is not covered by the sheet when the leading edge of the sheet is nipped by the conveyance roller. After the leading edge of the sheet has been nipped, the sticking and conveying section can be stopped, but the sheet is required to be conveyed by the sticking and conveying section until the moment when the leading edge of the sheet is nipped by the conveyance roller. During the period when the sheet is conveyed by the sticking and conveying section, the sticking duct is required to continue sticking. In that case, a portion of the sucking duct which is not covered by the sheet sucks the succeeding sheet to cause multi-feeding, during the period from the moment when a trailing edge of a sheet passed the rear end of the sucking duct up to the moment when a leading edge of the sheet is nipped by the conveyance roller, which is a problem.
However, a method to solve this problem is not described in the aforesaid Unexamined Japanese Patent Application Publications Nos. 60-52428, 06-219578, 60-52429 and 05-270676. In this case, it is considered that a dimension of the sucking duct is made to be small, adjusting to the smallest size of a sheet conveyed by a sheet feeding device. However, when doing so, sucking power lacks and causes conveyance defects in the case of a large-sized sheet.